html{
  height: 100%;  
}
body {
  display: flex;
  flex-direction: column;
  height: 100%;
  background-image: url(../images/main_bg.png);
  background-color: #f5f5f5;
}

header {
  display: flex;
  justify-content: space-around;
  align-items: center;
  padding: 10px 0;
  flex: 0 0 auto;
}

main {
  flex: 1 0 auto;
}

footer {
  flex: 0 0 auto;
  text-align: center;
}